<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <div id="app">
  <input type="text" v-model.number="val1">
  <select name="" id="" value="" v-model="val3">
    <option value="+">+</option>
    <option value="-">-</option>
    <option value="*">*</option>
    <option value="/">/</option>
  </select>
  <input type="text" v-model.number="val2">
  =<span></span>
  </div>
  <script src="https://cdn.bootcdn.net/ajax/libs/vue/2.7.14/vue.min.js"></script>
  <script>
    const an = new Vue({
      el: '#app',
      data: {
        val1:'',
        val2:'',
        val3:''
      },
      methods: {
        total(){
          return 
        }
      }
    })
  </script>
</body>

</html>